Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for an image pickup optical system used for an image pickup apparatus using an image pickup element, such as a digital still camera, a video camera, a surveillance camera, an on-vehicle camera, or a broadcasting camera.
Description of the Related Art
In recent years, as an image pickup optical system used for a surveillance camera, an on-vehicle camera, or the like, a fisheye zoom lens having a wide angle of field, which facilitates image capturing with small dead angle, is required. The fisheye zoom lens is primarily intended to secure a wide angle of field by allowing a distortion. For example, there is known a type in which an image circle diameter is set with respect to a diagonal direction of an image pickup element at a lens image forming position, and in which a total angle of field (2ω) of the image pickup element is about 180 degrees diagonally (hereinafter referred to as “full-frame fisheye type”).
Meanwhile, there is also known a type in which a total angle of field in a short side direction of the image pickup element is about 180 degrees, and in which a substantially circular image is formed on the image pickup element of an image pickup apparatus to secure the angle of field of about 180 degrees circumferentially, which includes diagonally, horizontally, and vertically (hereinafter referred to as “circular fisheye type”).
In Japanese Patent Application Laid-Open No. 2012-22109, there is described a zoom lens capable of switching between the full-frame fisheye type and the circular fisheye type depending on a magnification-varying state. In Japanese Patent Application Laid-Open No. 2012-194238, there is disclosed a fisheye zoom lens, which has a half angle of field of about 90 degrees at a shortest focal length (wide angle end), and is capable of zooming to a standard angle of field at a telephoto end.
The circular fisheye type has problems in that not all pixels of the image pickup element are effectively used, and in that a magnification is reduced to result in insufficient ability to discern a subject. Therefore, for an image pickup apparatus such as the surveillance camera or the on-vehicle camera, a zoom lens having good optical performance over the entire range from a range including the circular fisheye type and the full-frame fisheye type to a longest focal length, at which the standard angle of field is obtained, is sought for.
In order to satisfy those requirements, it is important to appropriately set a shape and a size of an image pickup region of the image pickup element, a change in angle of field and a change in maximum image height accompanying zooming of the zoom lens, and other such factors with respect to the image pickup region of the image pickup element.